1. Field of the Invention
This invention is in the field of chemical watermarks, i.e., marks that are laid down upon paper products to provide a characteristic design to convey intelligence.
2. DESCRIPTION OF THE PRIOR ART
In the office operations of commercial enterprises, it is often desirable to have writing stationery and other forms of business and professional papers provided with a watermark. These watermarks are sometimes business trandemarks used to identify the manufacturer of the paper product or they may contain coded information that relates to dates of manufacture or composition. Sometimes business trademarks are watermarked for identity and protective purposes and as an additional means of distinctively identifying a particular business enterprise.
The usual practice of watermarking is performed mechanically during the papermaking process by providing rolls with raised designs interposed usually at a point before the wet paper web enters the driers. The fibers in the web are displaced in the horizontal direction and the paper is rendered thinner and consequently denser and more transparent in the area of the watermark when viewed under transmitted light. Thus, the embossed rolls form a configuration or design of the mark in the damp or wet paper which remains permanent upon drying. After the papermaking and watermarking process is complete, the finished paper may be cut into sheets of various sizes, for use as stationery or business papers of the like.
Another known method of mechanically producing a watermark involves employing a band fitted about a roll and carrying the design or configuration of the watermark in relief. The design of the mark in relief on the band is brought into contact with the wet paper web at the wet presses beyond the Fourdrinier wire. At that point the paper is still plastic and compresses readily. The resulting compression of fibers reduces entrapped air and light refraction, and increases the transparency in the area conforming to the configuration of the watermark.
Mechanically produced watermarked paper has several disadvantages. For one, the commercial papermaking machines are very large and produce paper at a high rate of speed so that for reasons of economy a customer desiring paper bearing a private watermark must purchase a relatively large amount of each kind of paper needed. This, of course, limits such privately watermarked papers to a few but relatively large industrial users or governmental departments since it is apparent that smaller enterprises could not economically purchase and stock such large supplies of paper products. Secondly, the cost of the embossed or banded rolls for the papermaking machine is obviously extremely high and could not, therefore, be afforded by smaller industrial users. Third, the cost of setting up the embossed or banded rolls involves both labor as well as the loss of production on the papermaking machinery and would have to be included in the net cost to the purchaser. Among other disadvantages, the purchaser finds that his private watermark is not positioned uniformly in the finished cut stationery unless he assumes the additional labor cost and paper waste involved in constantly positioning the relative position of the rolled paper and the cut-off knife. While private watermarks may be formed by the mechanical methods outlined above, it is thus clear that orders for paper products so marked must be large if the cost thereof is to be economically justified, so that orders in small quantities are of such high cost as to be rarely justified.
There have been some disclosures of application of chemical watermarks to otherwise finished paper products. Generally, these processes have in common the steps of applying a chemical material in the form of a solution in a volatile solvent to the otherwise finished paper product, allowing or otherwise causing the solution to penetrate into the fiber structure of the sheet, and then removing the volatile solvent. The solvent may be removed by evaporation or normal drying at ambient temperatures or by forced drying with the application of heat or forced air streams. One of the earliest of such disclosures appears in Livingstone U.S. Pat. No. 3,048,100 which is directed specifically to a chemical watermark produced by treating the paper with a cyclohexanone resin dissolved in a volatile solvent.
Vaurio U.S. Pat. No. 3,085,898 to an impregnating solution containing about 30 to 70% by weight of either sucrose acetate butyrate, sucrose acetate isobutyrate or an alkali metal salt of a dialkyl sulfosucinate dissolved in an inert organic solvent for chemical watermarking.
Vaurio U.S. Pat. No. 3,140,959 refers to the use of synthetic, thermosetting, curable resins in solution in volatile organic solvents in the formation of chemical watermarks. In one embodiment of the invention, up to 60% by weight of curing agents for the thermosettable resin are employed to render the resin insoluble in water and common solvents. In another embodiment of the invention, prehardened synthetic thermosetting resins in solution in volatile organic solvents are utilized to form the chemical watermark.
Schur U.S. Pat. No. 3,288,628 describes a chemical water-marking process employing a liquid organic viscous ester having a molecular weight in the range of about 300 to 10,000 in combination with a monomeric non-volatile liquid organic ester plasticizer.
Skofronick et al. U.S. Pat. No. 3,293,062 describes the use of a curable hexamethoxymethyl melamine and a sulfonic acid derivative as a curing agent in the formation of chemical watermarks. The watermark products of this invention generally require 5 to 15 days aging at ambient temperatures in order to render the watermark insoluble in water, ethanol or other common solvents.
While the disclosures mentioned above can be used to produce chemical watermarks, many of them have disadvantages of their own. Some produce watermarks which are leachable from the paper, or become smeared, or otherwise distorted by the action of water or chemical solvents on the paper products containing such watermarks. Since a variety of printing inks and printing processes in common use today utilize water and chemical solvents in their operation, they present the possibility of attacking such watermarks, distorting their original form, and adversely affecting their distinctiveness.
In some cases, the processes employed make use of curing agents which are noxious and unpleasant chemical compounds or give rise to unpleasant and noxious vapors or byproducts upon being heated to effect the curing reaction. In some instances, the curing agents are vesicants and skin sensitizing agents which must be handled with extreme care to avoid chemical burns or contact dermatitis. In other cases, the compositions require excessively long periods of time for the curing reaction to occur so that the chemical watermark so produced would not be undesirably altered by contact with water or common organic solvents. In some instances, the curing agents are highly acidic compounds which could tend to lower the strength properties of the paper in the area of the watermark upon long-term aging or exposure to higher than normal temperatures. This type of degradation is normally accompanied by embrittlement and loss of physical strength characteristics. The curing reaction which renders the watermark insensitive to the water and common solvents may be effected in a shorter time span if the paper is heated to an elevated temperature after application of the chemicals but the obvious increased cost and increased degree of control which must be exercised over this type of process makes this option expensive and disadvantageous.